The present invention relates to a method and device for controlling a diesel engine with a forced induction system.
In diesel engines mounted in automobiles, a plurality of fuel injections are performed within cylinders during one cycle of the engine in order to, for example, reduce NOx and soot contained in exhaust gas, reduce noises or vibrations, and improve a fuel consumption and torque. For example, JP2009-293383A discloses a diesel engine with a turbocharger in which fuel injection is performed at five timings: a main injection for generating a torque, a pilot injection performed prior to the main injection so as to preheat cylinders, a pre-injection performed between the pilot injection and the main injection to suppress an ignition delay of fuel injected by the main injection, an after injection performed after the main injection so as to raise a temperature of exhaust gas, and a post injection for raising a temperature of a catalyst by directly introducing the fuel to an exhaust system subsequently to the after injection.
Further, for example, JP2005-240709A discloses an art in which, in view of increasing temperatures inside cylinders by preliminary combustion prior to main combustion, an injection amount of fuel by a pilot injection is changed according to an engine load and an engine speed. Thereby, the temperatures inside the cylinders are increased at the time of performing a main injection to surely exceed a temperature in which the fuel can self-ignite, and thus, misfire of the fuel injected by the main injection is prevented.
Meanwhile, in a diesel engine where fuel supplied to cylinders self-ignites through compression, a comparatively low compression ratio in which a geometric compression ratio is, for example, 15:1 or lower is set in order to achieve, for example, a reduction of a discharge amount of NOx. The low compression ratio subsides the combustion in each of the cylinders and suppresses generation of NOx. Further, because the lowered compression ratio of the engine reduces a mechanical resistance loss, it is also beneficial in improving a thermal efficiency of the engine.
Generally, in view of improving a fuel consumption and an NVH performance of the diesel engine, main combustion mainly including diffusion combustion is desired to stably be performed near a top dead center of a compression stroke. However, after the injection of the fuel, if an ignition delay until a start of the combustion becomes long, a controllability of the combustion degrades and, therefore, it becomes difficult to, for example, stably perform the main combustion near the top dead center of the compression stroke. The inventors have found that performing a pre-stage injection at least once prior to a main injection to cause pre-stage combustion appropriately before the start of the main combustion shortens the ignition delay comparatively by optimizing cylinder internal temperature and pressure at the start of the main combustion and benefits in improving the controllability of the main combustion, in other words, improves a fuel consumption and an NVH performance.
However, the lowered compression ratio of the engine decreases the temperature and the pressure at the end of the compression stroke when the engine speed and the engine load are low, and therefore, in order to increase the cylinder internal temperature and pressure to improve the controllability of the main combustion, a large amount of fuel needs to be injected by the pre-stage injection. For example, particularly, when the engine is in a non-warmed-up state or an outside air temperature is low where the temperature at the end of the compression stroke decreases, or under a high altitude condition where the temperature and pressure at the end of the compression stroke decrease, the fuel injection amount by the pre-stage injection needs to significantly be increased.